In the fabrication of light-emitting diodes, III-nitride-based semiconductors, such as GaN, AlGaN, InGaN and AlInGaN, are common. Usually, epitaxial structures of most of the light-emitting devices made of the III-nitride-based semiconductors are grown on an electrically insulating sapphire substrate, which is different from other light-emitting devices utilizing conductive substrates. The sapphire substrate is an insulator, so an electrode cannot be directly formed on the sapphire substrate. Electrodes have to be formed to contact respectively a p-type semiconductor layer and an n-type semiconductor layer directly, so that the light-emitting devices of the aforementioned type can be completed.
Referring to FIGS. 1(a) and 1(b), FIG. 1(a) illustrates a top view of a conventional light-emitting diode chip, and FIG. 1(b) illustrates a cross-sectional view of the light-emitting diode chip along the cross-sectional line B-B′ shown in FIG. 1(a). The conventional light-emitting diode 200 is mainly composed of a transparent substrate 202, an epitaxial structure located on the transparent substrate 202 and two electrodes, in which the epitaxial structure principally includes a first conductivity type semiconductor layer 204, an active layer 206 and a second conductivity type semiconductor layer 208 stacked in sequence. The first conductivity type semiconductor layer 204 is deposed on the transparent substrate 202, the active layer 206 is deposed on a portion of the first conductivity type semiconductor layer 204 to expose the other portion of the first conductivity type semiconductor layer 204, and the second conductivity type semiconductor layer 208 is deposed on the active layer 206. A transparent conductive layer 210 is provided on a portion of the second conductivity type semiconductor layer 208 for the improvement of current spreading. A second conductivity type electrode pad 212 is deposed on the exposed portion of the second conductivity type semiconductor layer 208 and a portion of the transparent conductive layer 210, and a stacked structure composed of a first conductivity type electrode 214 and a first conductivity type electrode pad 216 is deposed on a portion of the exposed portion of the first conductivity type semiconductor layer 204, such as shown in FIG. 1(b).
With respect to the structure illustrated in FIG. 1(b), when an area of the first conductivity type semiconductor layer 204 is exposed by removing a portion of the epitaxial structure for deposing the first conductivity type electrode 214 and the first conductivity type electrode pad 216, an etching process is performed and stopped at the first conductivity type semiconductor layer 204. The conventional light-emitting diode 200 cannot spread current because the two electrode structures of the light-emitting diode 200 are typically on the diagonal line of the light-emitting diode chip, which easily causes excessive current density in a local area. Accordingly, when the operating current is increased, because the current distribution between the first conductivity type electrode pad 216 and the second conductivity type electrode pad 212 is not uniform, the region A of the light-emitting diode 200 shown in FIG. 1(a) is easily damaged or the efficiency of the light-emitting diode 200 is reduced by the excessive current density.
In order to improve the aforementioned issue of the conventional light-emitting diode structure, a light-emitting diode 300 such as illustrated in FIG. 2 is provided in U.S. Pat. No. 6,307,218 by Lumileds of the United States of America. A first conductivity type electrode 304 of the light-emitting diode 300 is deposed on the exposed portion of a first conductivity type semiconductor layer 302, a second conductivity type electrode 308 is deposed on a transparent electrode 306, and most of the first conductivity type electrode 304 and the second conductivity type electrode 308 are parallel to improve the distribution of current. Although the light-emitting diode 300 has parallel electrodes, current cannot be uniformly spread at the electrode edges, such as at a region B and a region C.
For example, if the light-emitting diode shown in FIG. 1(b) is a green light LED (having a wavelength of 525 nm) 14 mil long×14 mil wide, the efficiency of the LED is 50 lm/W; if the green LED is 40 mil long×40 mil wide and is designed as the structure shown in FIG. 2, the efficiency of the LED is lowered to 35 lm/W. As the dimensions are decreased, the efficiency of the LED is decreased, as shown in FIG. 3. Therefore, the parallel electrodes cannot enhance the uniformity of current effectively.
Accordingly, because electrodes of a conventional light-emitting diode are usually deposed on the diagonal line of the light-emitting diode chip, and the etching process used to remove a portion of the epitaxial structure is typically stopped at the first conductivity type semiconductor layer, excessive current density in the local area is easily caused. Particularly, when the light-emitting diode is operated at high power, the area within the shortest path between electrodes is easily damaged by the excessively concentrated current, and the efficiency of the light-emitting diode is decreased with increasing operating power. Accordingly, it is desirable to provide a light-emitting diode without the above shortcomings.